Since the late 1980's, elastomer coated stainless steel coils have been used to design multi-layered steel engine head gaskets. In particular, these stainless steel coils have been coated with elastomers such as nitrile butadiene rubber or fluoroelastomer. Application of a thin primer between the rubber coating and stainless steel surface is a common industrial practice since an elastomer coating alone does not provide the desired wet adhesion on stainless steels. Generally speaking, the resistance of the primer to the surrounding media, such as engine oil, coolant, fuel and heat, in a great degree, decides the performance and durability of the gasket.
Numerous silane adhesive compositions have previously been developed for the purpose of bonding elastomeric materials to various substrates. For example, U.S. Pat. Nos. 5,532,314 and 5,907,015 relate to silane adhesive compositions which have various organic-functional silanes as a bonding agent. Primers based on silanes generally show good dry adhesion in bonding organic coatings to steel or other metal substrates. However, some of the silane based primers have been found to not have the desired durability in hot coolant immersion.
Phenolic resoles toughed by elastomers or flexible organic resins are widely used as primers or structural adhesives to bond metals. The adhesion between the phenolic resole based primers and the metal surface can be enhanced by addition of organic silanes. In many applications, satisfactory adhesion is achieved using a phenolic/silane based primer. However, the adhesion is gradually lost under aggressive wet environments such as boiling water exposure or hot coolant immersion.
Thus, there is a need in the art for a primer or adhesive composition for bonding elastomers to substrates which exhibits good adhesion in both dry and wet applications. There further is a need in the art for a primer or adhesive composition which shows robust dry and wet adhesion to metallic or other surfaces under harsh environments such as hot oil, hot coolant and thermal aging. Additionally, there is a need in the art for a coolant resistant primer or adhesive for bonding elastomers to substrates which also is thermally stable. Moreover, there is a need in the art for a primer composition which can bond elastomers to substrates without surface treatment of the substrate. This invention provides such a primer and a method for bonding an elastomer to a substrate with the primer of the invention.